9.15.4 i386-Mnemonics

9.15.4.1 Instruction Naming

Instruction mnemonics are suffixed with one character modifiers which
specify the size of operands. The letters ‘b’, ‘w’, ‘l’
and ‘q’ specify byte, word, long and quadruple word operands. If
no suffix is specified by an instruction then as tries to
fill in the missing suffix based on the destination register operand
(the last one by convention). Thus, ‘mov %ax, %bx’ is equivalent
to ‘movw %ax, %bx’; also, ‘mov $1, %bx’ is equivalent to
‘movw $1, bx’. Note that this is incompatible with the AT&T Unix
assembler which assumes that a missing mnemonic suffix implies long
operand size. (This incompatibility does not affect compiler output
since compilers always explicitly specify the mnemonic suffix.)

Almost all instructions have the same names in AT&T and Intel format.
There are a few exceptions. The sign extend and zero extend
instructions need two sizes to specify them. They need a size to
sign/zero extend from and a size to zero extend to. This
is accomplished by using two instruction mnemonic suffixes in AT&T
syntax. Base names for sign extend and zero extend are
‘movs…’ and ‘movz…’ in AT&T syntax (‘movsx’
and ‘movzx’ in Intel syntax). The instruction mnemonic suffixes
are tacked on to this base name, the from suffix before the
to suffix. Thus, ‘movsbl %al, %edx’ is AT&T syntax for
“move sign extend from %al to %edx.” Possible suffixes,
thus, are ‘bl’ (from byte to long), ‘bw’ (from byte to word),
‘wl’ (from word to long), ‘bq’ (from byte to quadruple word),
‘wq’ (from word to quadruple word), and ‘lq’ (from long to
quadruple word).

are called ‘cbtw’, ‘cwtl’, ‘cwtd’, ‘cltd’, ‘cltq’, and
‘cqto’ in AT&T naming. as accepts either naming for these
instructions.

Far call/jump instructions are ‘lcall’ and ‘ljmp’ in
AT&T syntax, but are ‘call far’ and ‘jump far’ in Intel
convention.

9.15.4.2 AT&T Mnemonic versus Intel Mnemonic

as supports assembly using Intel mnemonic.
.intel_mnemonic selects Intel mnemonic with Intel syntax, and
.att_mnemonic switches back to the usual AT&T mnemonic with AT&T
syntax for compatibility with the output of gcc.
Several x87 instructions, ‘fadd’, ‘fdiv’, ‘fdivp’,
‘fdivr’, ‘fdivrp’, ‘fmul’, ‘fsub’, ‘fsubp’,
‘fsubr’ and ‘fsubrp’, are implemented in AT&T System V/386
assembler with different mnemonics from those in Intel IA32 specification.
gcc generates those instructions with AT&T mnemonic.